Solar Cell Having Doped Semiconductor Heterojunction Contacts

ABSTRACT

A silicon solar cell has doped amorphous silicon contacts formed on a tunnel silicon oxide layer on a surface of a silicon substrate. High temperature processing is unnecessary in fabricating the solar cell.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/381,681, filed on May 4, 2006, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates generally to photovoltaic solar cells, and moreparticularly the invention relates to a solar cell structure which isefficient in operation and economical to manufacture.

The use of photovoltaic cells for the direct conversion of solarradiation into electrical energy is well known, see Swanson, U.S. Pat.No. 4,234,352 for example. Briefly, the photovoltaic cell comprises asubstrate of semiconductive material having a p-n junction definedtherein. In the planar silicon cell the p-n junction is formed near asurface of the substrate which receives impinging radiation. Radiatedphotons create mobile carriers (holes and electrons) and the substratewhich can be directed to an electrical circuit outside of the cell. Onlyphotons having at least a minimum energy level (e.g., 1.1 electron voltfor silicon) can generate an electron-hole pair in the semiconductorpair. Photons having less energy are either not absorbed or are absorbedas heat, and the excess energy of photons having more than 1.1 electronvolt energy (e.g. photons have a wavelength of 1.1 μm and less) createheat. These and other losses limit the efficiency of siliconphotovoltaic cells in directly converting solar energy to electricity toless than 30%.

Solar cells with interdigitated contacts of opposite polarity on theback surface of the cell are known and have numerous advantages overconventional solar cells with front side metal grids and blanket or gridmetallized backside contacts, including improved photo-generation due toelimination of front grid shading, much reduced grid series resistance,and improved “blue” photo-response since heavy front surface doping isnot required to minimize front contact resistance because there are nofront contacts. In addition to the performance advantages, theback-contact cell structure allows simplified module assembly due tocoplanar contacts. See Swanson U.S. Pat. No. 4,927,770 for example.

While interdigitated back-contact (IBC) solar cells have beenfabricated, cost considerations have limited commercialization of theIBC solar cell. Heretofore, conventional microelectronics (integratedcircuit) processing has been employed in fabricating IBC solar cells,including the use of backside diffusions, contacts, and metal linesfabricated by conventional microelectronics photolithography, thin filmmetallization, and etching processes. This fabrication process iscapable of producing high efficiency solar cells, but the process is notcost effective for application in conventional low-cost, flat-platesolar panels. The key problem with practical realization of an IBC solarcell by this process is the high cost of fabrication, including etching,doping and mask alignment, and the use of thick metal conductordeposition by vacuum evaporation or sputtering. Further, the processingmust be carried out in a clean room environment. Thus IBC solar cellsfabricated using these methods have been restricted to application inhigh concentration solar cells or in very high value one-sunapplications.

Copending application Ser. No. 11/306,510 combines a semiconductorsubstrate with acceptor and donor polymer contacts to provide a solarcell which is economically fabricated. Importantly, fabrication of thesolar cell, is improved in cost and in reduced temperature cyclingthrough use of inkjet application of the polymer contacts without theneed for photoresist masking, etching, and dopant diffusion andannealing as is required in prior art solar cells.

The present invention utilizes a semiconductor such as amorphous siliconas donor and acceptor contact in a silicon solar sell which can bereadily and cost effectively fabricated.

SUMMARY OF THE INVENTION

The invention utilizes doped amorphous silicon, Si—Ge, or III-Vcompounds as a donor or an acceptor contact in silicon solar cell. Thecontact material can be vapor deposited along with the dopant asnecessary for donor or acceptor application. As used herein, “amorphous”silicon includes “poly crystalline” silicon.

When deposited on a single crystal silicon substrate, a tunnel oxide isfirst grown and separates a deposited amorphous silicon from thesubstrate to prevent re-crystallization of the amorphous silicon.

In an interdigitated back contact (IBC) cell, the front surface can betextured by chemical or physical abrasion to provide a radiationcapturing surface with an anti-reflective and passivating coating suchas silicon nitride, doped silicon carbide, or a thin coating ofamorphous silicon over the textured surface.

The invention and object and features thereof will be more readilyapparent from the following detailed description and appended claimswhen taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view in section of an interdigitated back contact solarcell including doped amorphous contact in accordance with one embodimentof the invention.

FIGS. 2A-2D are side views in section illustrating the solar cell ofFIG. 1 during fabrication.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with one embodiment of the invention, an interdigitatedback contact (IBC) solar cell comprising a silicon semiconductor bodyhaving first and second opposing major surfaces receives radiationthrough the first surface and has first and second patterns of acceptordoped amorphous silicon and donor doped amorphous silicon, respectively,on the second or back surface for receiving electron and hole carrierscreated in the silicon substrate by radiated photons. The structure issimilar to prior art back contact solar cells which utilize doped P andN conductivity contacts formed in the substrates for receiving the holesand electrons created by radiation. However, the use doped P and Ncontacts in the substrate requires photoresist masking, etching, dopantdiffusion, and high temperature processing in the fabrication of thesolar cell. The use of acceptor and donor amorphous silicon contacts onthe structure, in accordance with an embodiment of the invention,obviates the need for photoresist masking and dopant diffusion and thehigh temperature processing required in annealing the diffused dopants.A tunnel silicon oxide can be placed between the contacts and thesubstrate to prevent epitaxial growth of the amorphous silicon on thesubstrate.

Consider now FIG. 1 which is a side view in section of an interdigitatedback contact (IBC) solar cell in accordance with one embodiment of theinvention. The cell includes a light n-type monocrystalline orpolycrystalline substrate 10 having a front surface which receivesradiation and a textured surface on which is formed a thin (e.g. 10-150angstrom) tunnel silicon oxide layer 12 with a passivation coating 14over tunnel oxide 12 which can comprise silicon nitride, doped siliconcarbide, or a doped amorphous silicon layer.

On the back surface of substrate 10 is a second tunnel oxide layer 16over which is formed P+ amorphous silicon contacts 18. A dielectric suchas silicon oxide 20 separates P+ amorphous silicon 18 from N+ amorphoussilicon 22 which is formed in openings through P+ amorphous siliconlayer 18 and in contact with tunnel oxide 16. While the amorphoussilicon layers 18, 22 are formed by low temperature vapor deposition,tunnel oxide 16 prevents any re-crystallization of the amorphous siliconby epitaxial growth from silicon substrate 10. Metal contact 24 engagesP+ amorphous silicon layer 18, and metal contacts 26 engage N+ amorphoussilicon layers 22.

The back contact heterojunction enhances the rear passivation of thesolar cell by the inclusion of tunnel oxide 16, heterojunction fieldprovided by the amorphous silicon contacts, and contact passivation. Aswill be described further herein below, a process benefit in making thedevice is that high temperature dopant drive is not required.

FIGS. 2A-2D are section views illustrating the solar cell of FIG. 1during fabrication. Initially, as shown in FIG. 2A, silicon substrate10, which can be either intrinsic or light doped, has a thin tunneloxide 16 grown thereon which can have a thickness of from 10-20angstroms, for example. Amorphous silicon layer 18 is then depositedwith a boron dopant and a dopant concentration of 10²⁰-10²¹ or10E20-10E21 atoms per cubic centimeter and to a thickness of 500 to 2000angstroms. The growth of a doped silicon layer by vapor deposition is aknown silicon process. Inclusion of an intrinsic layer under the p-typedoped silicon layer can be made if a PIN structure is desired.

Thereafter, an insulating layer of silicon oxide 20 is deposited by lowpressure chemical vapor deposition (LPCVD, PECVD, APCVD), or by a spinon glass process. Silicon oxide layer 16 is 500 to 1000 angstroms inthis illustrative embodiment.

Thereafter, as illustrated in FIG. 213, the front surface of substrate10 is textured by chemical or mechanical abrasion. This process step canprecede the process steps of FIG. 2A, if desired. A photoresist mask isthen formed on the back surface of the substrate 10 and etched to formopenings through silicon oxide layer 20 and amorphous silicon 18 tosubstrate 10. The thin tunnel oxide is removed in the etching processalso, and a new layer of tunnel silicon oxide is then applied on theexposed surface of substrate 10 through the etched openings by chemicalgrowth. In forming tunnel oxide 16 in FIG. 213, tunnel oxide layer 12can be simultaneously formed on the front surface. Following the growthof the thin tunnel oxide in the etched openings, again to a thickness of10-20 angstroms, an N+ doped amorphous silicon layer 22 is depositedover the back surface, as shown in FIG. 2C.

The layer 22 is doped with an N dopant such as phosphorus with aconcentration of 10²⁰-10²¹ or 10E20-10E21 atoms per cubic centimeter.This can be deposited using plasma enhanced chemical vapor deposition(PECVD, LPCVD, APCVD). N+ amorphous silicon 22 is then masked andselectively etched to expose the underlying P+ amorphous silicon 18 forreception of metal contacts. In FIG. 2D, metal contacts 24 and 26 aremade to P+ amorphous silicon 18 and N+ amorphous silicon 22 by metaldeposition and photoresist masking and etching. The contacts can beformed by first scattering a seed layer of a conductive metal such asaluminum or copper and then pattern plating the seed metal to increasethickness. The cell is then completed by depositing a passivating layer14 on tunnel oxide 12 on the front surface of substrate 10 using siliconnitride, doped silicon carbide, or N+ doped amorphous silicon.

A heterojunction solar cell in accordance with the invention, usingdoped amorphous silicon contacts is readily fabricated usingconventional semiconductor processing techniques without the for hightemperature processing. While the invention has been described withreference to an interdigitated back contact solar cell in which both P+and N+ contacts are employed, the invention can be applied to solarcells having a single doped amorphous silicon on the back surface.Further, while the heterojunction is provided by amorphous silicon,other high band gap material such as germanium-silicon alloy, dopedsilicon carbide, or other III-V compound material can be employed in thecontact structures. Thus, while the invention has been described withreference to specific embodiments, the description is illustrative ofthe invention and is not to be construed as limiting the invention.Various modifications and applications may occur to those skilled in theart without departing from the spirit and scope of the invention asdefined by the appended claims.

1-10. (canceled)
 11. A solar cell comprising: a silicon substrate inwhich mobile carriers are created by impinging radiation; a contact forreceiving mobile carriers comprising a dielectric layer on a surface ofthe substrate, and a doped semiconductor material layer on thedielectric layer.
 12. The solar cell of claim 11 wherein thesemiconductor material is selected from the group of consisting ofamorphous silicon, silicon-germanium, and III-V compoundssemiconductors.
 13. The solar cell of claim 12 wherein the semiconductormaterial comprises amorphous silicon.
 14. The solar cell of claim 13wherein the dielectric layer comprises silicon oxide.
 15. The solar cellof claim 14 wherein the silicon oxide has a thickness in the range of10-20 angstroms.
 16. The solar cell of claim 14 wherein the amorphoussilicon is doped with a donor dopant.
 17. The solar cell of claim 16wherein the donor dopant is phosphorus.
 18. The solar cell of claim 14wherein the amorphous silicon is doped with an acceptor dopant.
 19. Thesolar cell of claim 18 wherein the acceptor dopant is boron.
 20. Amethod of fabricating a silicon solar cell with carrier acceptorcontacts, the method comprising: a) providing a silicon substrate havingfirst and second opposing major surfaces, wherein the first majorsurface is a light incident side; b) forming a dielectric layer on thesecond major surface; and c) forming a doped amorphous silicon layer onthe dielectric layer.
 21. The method of claim 20, wherein dopedamorphous silicon has dopant in excess of 10¹⁹ atoms per cc.
 22. Themethod of claim 21, wherein the dopant is a donor dopant.
 23. The methodof claim 22, wherein the dopant comprises phosphorus.
 24. The method ofclaim 21, wherein the dopant is an acceptor dopant.
 25. The method ofclaim 24, wherein the dopant comprises boron.
 26. The method of claim 20wherein the dielectric layer has thickness on the order of 10-20angstroms.
 27. The method of claim 20, wherein the dielectric layercomprises silicon oxide.
 28. The method of claim 27, wherein thedielectric layer comprises a tunnel silicon oxide layer.
 29. The solarcell of claim 11, wherein the dielectric layer comprises a tunnelsilicon oxide layer.